A traditional reflective photoelectric sensor detects whether or not there is an object by a change in the magnitude of optical feedback that is produced through a reflection on an object. Because of this, when the reflectivity of the background is large and the reflectivity of the object is small when comparing the reflectivities of the object and the background, there will be cases wherein the background is detected as the object. There are photoelectric sensors of a polarized type (in, for example, Japanese Unexamined Patent Application Publication H6-111693) and photoelectric sensors of a background setting reflective type (background suppression type, hereinafter abbreviated “BGS”) as reflective photoelectric sensors that are less susceptible to the effects of the background. (See, for example, Japanese Unexamined Utility Model Registration Application Publication S63-102135 (“JP '135”) and Japanese Unexamined Utility Model Registration Application Publication S63-187237 (“JP '237).
In the polarized-type photoelectric sensors, a light projecting/receiving device and a reflex reflector are disposed facing each other, and a polarizing filter for emitting polarized light having a specific plane of polarization is disposed at the front surface of the light projecting device, and a polarizing filter for receiving polarized light having a plane of polarization that is perpendicular to that of the light projecting device side is disposed at the front surface of the light receiving device. The reflex reflector is a corner cube set, wherein the incident light is returned in the same direction as the incident light after being completely reflected three times.
The light that is emitted from the light projecting device becomes linearly polarized light through passing through the polarizing filter on the light projecting device side. When this linearly polarized light is incident into the reflex reflector, the reflected light becomes elliptically polarized light, producing a polarization component that is perpendicular to the incident light. Consequently, this polarization component passes through the polarizing filter on the light receiving device side to be incident into the light receiving device. On the other hand, when an object incurs between the light projecting/receiving device and the reflex reflector, light that is linearly polarized in the same direction as the incident light is reflected. This linearly polarized light cannot pass through the polarizing filter on the light receiving device side, and so is not incident into the light receiving device. In this way, the polarized photoelectric sensor is able to detect whether or not there is an object based on whether or not there is light incident into the light receiving device.
The BGS photoelectric sensor detects whether the distance of an object is more distant or closer than a predetermined reference distance by measuring the distance of the object optically.
On the other hand, there have been proposals for laser measuring devices that use interference (the self-coupling effect) within the semiconductor laser between the laser output light and the optical feedback from the measurement object as distance meters using interference of the light from a laser. (See, for example, UEDA Tadashi, YAMADA Jun, SHITO Susumu: “Distance Measurement Using the Self-Coupling Effect of Semiconductor Lasers,” 1994 Annual Joint Conference Lecture Proceedings of the Tokai Branch of the Electrical Society, 1994; YAMADA Jun, SHITO Susumu, TSUDA Norio, UEDA Tadashi: “Research regarding Small Distance Meter Using the Self-Coupling Effect of Semiconductor Lasers,” Aichi Institute of Technology Research Reports, 31B, Pages 35-42, 1996; and, Guido Giuliani, Michele Norgia, Silvano Donati, and Thierry Bosch, “Laser Diode Self-Mixing Technique for Sensing Applications,” Journal of Optics A: Pure and Applied Optics, Pages 283 through 294, 2002.) An FP-type (Fabry-Perot type) semiconductor laser compound resonator model is illustrated in FIG. 8. In FIG. 8: 101 is a semiconductor laser; 102 is a wall open surface in the semiconductor crystal; 103 is a photodiode; and 104 is a measurement object.
When the laser oscillation wavelength is defined as λ and the distance to the measurement object 104 from the wall open surface 102 that is closer to the measurement object 104 is defined as L, then the laser output becomes slightly increased due to the reinforcing of the laser light within the resonator 101 with the optical feedback from the measurement object 104:L=qλ/2  (1)
In Equation (1), q is an integer. This phenomenon can be fully observed through the occurrence of an amplifying effect through the amplification of the apparent reflectivity within the resonator 101 of the semiconductor laser, even if the scattered light from the measurement object 104 is extremely weak.
The semiconductor laser emits laser light that will vary in the frequency depending on the magnitude of the injected electric current, so that it is possible to modulate the laser directly, through the injected electric current, without requiring an external modulating device, when modulating the oscillation frequency. FIG. 9 is a diagram illustrating the relationship between the oscillation wavelength and the output waveform of the photodiode 103 when the semiconductor laser oscillating wavelength is varied at a constant rate. When the L=q λ/2 indicated in Equation (1) is satisfied, then there will be a phase difference of 0° (that is, equal phases) between the optical feedback and the laser light within the resonator 101, where the optical feedback and the laser light within the resonator 101 will reinforce each other most strongly, and when L=q λ/2+λ/4, then the phase difference will be 180° (opposite phases), so that the laser light in the optical feedback resonator 101 will weaken each other the most. Because of this, places where the laser output becomes stronger and places wherein the laser output becomes weaker will appear repetitively alternatingly as the oscillation wavelength of the semiconductor laser is varied, at which time the detection of the laser power by the photodiode 103 that is provided in the resonator 101 can produce a waveform that undergoes steps at specific periods, as illustrated in FIG. 9. This type of waveform is typically known as an interference fringe.
In this stepped waveform, the individual interference fringes are known as mode hop pulses (hereinafter termed “MHPs). An MHP is a different phenomenon from the mode hopping phenomenon. For example, when the distance from the measurement object 104 is L1, then if the number of MHPs is 10, then at half the distance, L2, the number of MHPs would be 5. That is, when there is a change in the oscillating wavelength of the semiconductor laser in a given time interval, then the number of MHPs will vary proportionately to the measurement distance. Consequently, the distance can be measured easily by measuring the frequency of the MHPs by detecting the MHPs using a photodiode 103.
The use of a self-coupling laser measuring device as set forth above enables a BGS photoelectric sensor to be achieved. The BGS photoelectric sensor may make an ON/OFF determination as to whether or not the object is at a near distance or at a far distance by comparing to a predetermined reference wavelength. Given this, when a self-coupling laser measuring device is used as a BGS photoelectric sensor, then the determination may be whether the average period of the measured MHP is longer or shorter than a known reference period of an MHP when the object is at the position of the reference distance. If the average period of the measured MHP is longer than the known reference period of the MHP when the object is positioned at the reference distance, then an ON determination is made that there is an object at a distance that is closer than then reference distance, and if the MHP period is shorter, then an OFF determination is made, as the object existing at a distance that is more distant than the reference distance.
As described above, in reflective photoelectric sensors that detect the presence or absence of an object based on the magnitude of the optical feedback, there is a problem in that there is a susceptibility to the influence of the background.
Additionally, in the polarized photoelectric sensor, even though there is no effect of the background because the sensor explicitly senses the background (the reflex reflector), it is possible, when there is an object with characteristics such as disrupting polarization, that it may be impossible to detect this object.Additionally, in a BGS photoelectric sensor, even though there is no effect from a background that exists at a distance that is more distant than any reference distance, if there is a mirrored surface in the background, or if the object is near to a mirrored surface, then there is the possibility that the operation will be destabilized due to the reflected light from the mirrored surface.
Additionally, in the BGS photoelectric sensor that uses a self-coupling laser measuring device, if the distance between the sensor and the background is close, then the magnitude of light returned from the background will be greater, but if the magnitude of the optical feedback is too large, then there will be a dramatic increase in the noise that is produced with in the laser, producing a coherent collapse phenomenon wherein the operation of the laser becomes unstable. Hence there will be the possibility of an error in the distance measurement due to an error in the MHP measurement, resulting in an incorrect determination.
Furthermore, in a BGS photoelectric sensor that uses a self-coupling laser measuring device, if the background is far, so that no MHPs occur through the self-coupling effect, or if it is not possible to obtain an adequate signal strength in the MHP detection, there will still be the possibility of the occurrence of noise, such as optical chaos, through the extremely small amounts of optical feedback from the background, and there is the possibility of an incorrect determination resulting from this noise.
The present invention was created in order to solve the problem areas set forth above, and the object thereof is to reduce the possibility of an incorrect determination, to provide a reflective photoelectric sensor and object detecting method wherein it is possible to detect an object reliably.